This project is investigating the nature of humoral antibodies which prevent induction of delayed hypersensitivity to purified antigens in mice, and the mechanisms by which they are effective. These antibodies appear to belong to an anodic subclass of 7S gamma l mouse globulins and may owe their activity to having a lower than usual pI. A rabbit antiserum specific for this subclass has been prepared and will be tested for capacity to specifically absorb these antibodies. If specific for the biologically active inhibitory antibodies, it will be used for studying their characteristics more rapidly, conveniently, and definitively than has been possible with hitherto used in vivo assays. Noninhibitory antibodies apparently can be converted to inhibitory by carbamylation to appropriately lower their pI. If this finding can be confirmed, its significance in development of delayed hypersensitivity and prevention of sensitization by especially anodic humoral antibodies will be investigated. The mechanisms by which anodic humoral antibodies known to prevent induction of delayed hypersensitvity in mice (so-called "contrasensitizing antibodies") function also will be investigated by independent collateral methods including observation of their effects on antigen and responding cells during induction of delayed hypersensitivity in vitro and by radiolabelling of antigen with a natural affinity for iron (chicken conalbumin), in vivo.